18 points is actually the lowest you can go, but people over 14 stone are advised not to go as low as that, think its 2 or 3 points less than your normal in that case.
I started two weeks ago with the kickstart diet and from what I understand from the booklet I was given you can do the kickstart for a maximum of two weeks. It is 18 points a day and if you're over 14 stones it's 22 points. Other than that there is no difference to the actual WW rules about points and saving points etc.
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I had guessed it was the 18 points a day thingie. My points work out at 19 a day, so not much benefit I think for me to do the Kick start. I am always reluctant to give up any of my points, even if its only 1.